This invention relates to an ion implantation apparatus for injecting an ion beam into a substrate.
On injecting an ion beam into a substrate or a wafer, the substrate is positioned on a disk section and is directed to the ion beam. When the ion beam is irradiated onto the substrate, a positive electric charge may be electrified on a surface of the substrate. Therefore, it is necessary to remove the positive electric charge from the substrate.
A conventional ion implantation apparatus is disclosed in Japanese Unexamined Patent Publication No. 10563/1989 (Tokkai Hei 1-10563) filed by the present assignee. In the conventional ion implantation apparatus, an ion beam is irradiated onto a substrate along a predetermined direction which may be called an irradiating direction hereinafter. Specifically, the conventional ion implantation apparatus comprises an electric conductor tube which is extended towards the irradiating direction and which has an inner wall. The inner wall is used as a target section for emitting electrons. An electron supplying section supplies primary electrons towards the target section to make the target section emit secondary electrons.
The secondary electrons are transferred in the form of an electron beam to the substrate through the electric conductor tube. As a result, it is possible to remove the positive electric charge from the substrate.
By the way, a secondary current is generated in the electric conductor tube when the secondary electrons are emitted from the target section. An amount of the secondary current is proportional to that of the secondary electrons. In addition, the amount of the secondary electrons is proportional to that of the primary electrons sent from the electron supplying section. This shows that the secondary current has to depend on the amount of the primary electrons. Therefore, it may be possible for the electron supplying section to substantially control the amount of the secondary electrons by detecting the amount of the secondary current.
However, undesired electrons are practically emitted from the substrate when the ion beam is irradiated to the substrate. The undesired electrons collide with an inner wall surface of the electric conductor tube. Such collision of the undesired electrons brings about a drastic change of the secondary current. As a result, it is practically difficult to suitably control the amount of the secondary electrons when a change of the secondary current is considered. In other words, the electron supplying section can not accurately control the amount of the secondary electrons. Therefore, it is difficult to avoid electrification of the substrate.